The invention relates to a fixing station and a method for the uniform fixing of toner images on a carrier material. The fixing station comprises a heating device having at least one heat radiation source emitting radiation toward the carrier material and a covering device with which an undesired incidence of radiation on the carrier material is prevented.
In electrographic printers or copiers, the toner image transferred from an intermediate carrier, generally a photoconductor, to the carrier material, generally paper, is fixed, i.e. it is joined to the carrier material in a smear and abrasion resistant way. Nowadays, heat-pressure fixing is commonly used in electrophotography. Without any preheating of the carrier material, for example with the aid of a heating saddle, this type of fixing is limited in its processing speed to about 0.5 m/s to 0.7 m/s. In the duplex printing mode, in which the front side and the rear or back side of a carrier material is printed, the fixing process is relatively difficult since still smearable toner images are applied to both sides. A high fixing quality together with a simultaneous fixing of the front side and the rear side of the carrier material can only be achieved with relatively soft fixing rollers, for example silicone rollers. These fixing rollers have a short life and are uneconomical. In addition, the guiding of the paper is problematic in the case of two opposite fixing rollers in particular when simultaneously fixing the front and the rear side of an endless carrier material.
U.S. Pat. No. 6,449,458 B1, whose disclosure is incorporated herein by reference thereto and which claims priority from German 198 27 210, discloses a fixing station for the simultaneous contact-free fixing of the front and the rear side of an endless carrier material with the aid of a heat radiation source. When fixing with the aid of this fixing station, a smearing of the not yet fixed toner images is avoided. In the case of the fixing station known from U.S. Pat. No. 6,449,458 B1, an additional covering device for the interruption of the ray path of the heat radiation source is provided, which covering device can be moved into the ray path between the heat radiation source and the carrier material. With the transport of the carrier material, the covering device is opened at the transporting speed of the carrier material, and after stopping the transport of the carrier material the covering device is closed in the opposite direction at the same speed.
Due to the operational sequence, known printers or copiers for printing endless carrier material are not only operated in the continuous printing mode but also in the intermittent printing mode. For example, the toner images of several color separations are transferred successively onto a transfer band, during which operation the transport of the carrier material is stopped. In addition, the transport of the carrier material is interrupted in the so-called start-stop mode, in the case of automatic cleaning processes and in the case of an interruption of the print data stream.
It is necessary that there is a smooth transition between a second printed image generated in a second printing process and a printed image generated in a first process. This requires a carrier material transport which, in particular, takes into account periods during which the transfer bands are swiveled to and away from the paper web as well as a predetermined acceleration and deceleration of the travel of the carrier material after every printing process. The acceleration and deceleration of the travel of the carrier material in a printing process are necessary for the synchronization with elements of the image generating unit of the printer or copier, in particular with a character generator, a photoconductor and a transfer element. In doing so, a backward pulling of the carrier material takes place, i.e. a transport in the opposite direction to the direction in which the carrier material is transported during printing. By means of this backward pulling of the paper, the positional displacements of the carrier material during the deceleration of the travel of the carrier material after the first printing process and the acceleration of the travel before the second printing process are compensated.
In the already mentioned fixing station, even in the case of a screening-off of the radiation of the heat radiation source as a result of the backward pulling of the carrier material, heat radiation is, however, once again supplied to an already completely fixed area of the carrier material by the opening of the covering device during the start of the carrier material transport in the second printing process. This results in visible differences in the fixed printed image. In addition, the stress on the carrier material varies, in particular additional moisture is extracted from a paper web by the additionally supplied radiation and as a result thereof, the paper web is further stressed.